1. Field of the Invention
The present invention relates to communications. More particularly, the invention concerns a method and apparatus for reducing search times associated with the handoff of a call from one base station to another base station.
2. Description of the Background Art
Wireless communication systems generally comprise, amongst other elements, a wireless unit, commonly referred to as a mobile telephone (mobile), that communicates with one or more base stations when making a call. The mobile communicates with the base stations on one or more channels that are contained within a frequency band assigned to the mobile by a base station controller. A communication from the mobile to a base station is made on what is called the “reverse link,” and a communication from the base station to the mobile station is made on the “forward link.” During a call, the mobile station is constantly searching for other base stations that the mobile might need to continue the call while the mobile station is moving around.
One important element of a mobile used in such a wireless system is the searcher. The searcher is programmed to search for pilot signals (pilots) transmitted from different bases stations in at least three cases: 1) when a mobile is trying to acquire a base station for communication; 2) in the idle state when the mobile is on the paging or access channels; and 3) in the traffic state where the mobile is in control of the traffic channel. The speed of searching the pilots on the frequency assigned to the mobile and other frequencies determines the search performance of the mobile. In slotted mode, the objective is to search all pilots in the neighbor set before the slot expires. Slotted mode refers to an operation mode of the mobile where the mobile monitors only during selected slots of time. Also, when searching pilots in a “candidate” frequency, the mobile needs to complete its search of all pilots in the candidate set as quickly as possible so that it tunes back to the serving frequency and minimizes the voice degradation caused by searching the candidate frequency. As discussed below, the candidate frequency is a potential handoff frequency, and these searching techniques are used to coordinate handoffs of communications in the wireless communication system.
A. Handoffs
A mobile used in a code-division-multiple-access (CDMA) wireless system supports three types of handoff procedures when the mobile is in control of the traffic channel. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990 and entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” assigned to the assignee of the present invention and incorporated by reference herein. The three types of handoffs are:                1. Soft Handoff—A handoff in which the mobile commences communications with a new base station without interrupting communications with the old base station. Soft handoff can only be used between CDMA channels having identical frequency assignments.        2. CDMA to CDMA Hard Handoff—A handoff in which the mobile is transitioned between disjoint sets of base stations, different band classes, different frequency assignments, or different frame offsets.        3. CDMA to Analog Handoff—A handoff in which the mobile is directed from a cdma forward traffic channel to an analog voice channel.To perform soft handoff, the mobile continuously searches for assigned sets of pilots. The term “pilot” refers to a pilot channel identified by a pilot sequence offset and a frequency assignment. A pilot is associated with the forward link traffic channels in the same forward link CDMA channel, or similarly with the reverse link on systems using reverse link pilots. All pilots in a pilot set have the same CDMA frequency assignment. For clarity, pilots are discussed in terms of the forward link only.        
The mobile searches for pilots on the current CDMA frequency assignment to detect the presence of CDMA channels and to measure their signal strength. When the mobile detects a pilot of sufficient strength that is not associated with any of the forward link traffic channels already assigned to it, it sends a pilot strength measurement message to the base station with which it is currently communicating. The base station can then assign a forward link traffic channel associated with that pilot to the mobile and direct the mobile to perform a handoff.
The pilot search parameters and the rules for pilot strength measurement message transmission are expressed in terms of the following sets of pilots:                Active Set: The pilots associated with the Forward Link Traffic Channels assigned to the mobile.        Candidate Set: The pilots that are not currently in the Active Set but have been received by the mobile with sufficient strength to indicate that the associated Forward Link Traffic Channels could be successfully demodulated.        Neighbor Set: The pilots that are not currently in the Active Set or the Candidate Set and are likely candidates for handoff.        Remaining Set: The set of all possible pilots in the current system on the current CDMA frequency assignment, excluding the pilots in the Neighbor Set, the Candidate Set, and the Active Set. This set of possible pilots consists of pilots whose pilot PN sequence offset indices are integer multiples of some pilot increment.The base station may direct the mobile to search for pilots on a different CDMA frequency to detect the presence of CDMA channels and to measure their strengths. The mobile reports the results of the search to the base station. Depending upon the pilot strength measurements, the base station can direct the mobile to perform an inter-frequency hard handoff.        
The pilot search parameters are expressed in terms of the following sets of pilots:                Candidate Frequency Neighbor Set: A list of pilots on the CDMA Candidate Frequency.        Candidate Frequency Search Set: A subset of the Candidate Frequency Neighbor Set that the base station may direct the mobile to search.        
B. Pilot Search
In current systems, the base station sets the search window, that is, the range of PN offsets, in which the mobile is to search for usable multipath components. These multipath components are used by the mobile for demodulation of an associated forward link traffic channel. Search performance criteria, and general wireless system criteria, are defined in standards TIA/EIA-95x and TIA/EIA-98-B, all issued by the Telecommunications Industry Association, and ANSI J-STD-018, issued by the American National Standards Institute, all of which are incorporated by reference herein. These searches are generally governed by the following:                Active Set and Candidate Set: The search procedures for pilots in the active and candidate sets are identical. The search window size for each pilot in the active and candidate sets is the number of PN chips specified in Table 1 corresponding to SRCH_WIN_A. For example, SRCH_WIN_AS=6 corresponds to a 28 PN chip search window or ±14 PN chips around the search window center. The mobile station centers the search window for each pilot of the active and candidate sets around the earliest arriving usable multipath component of the pilot.        
TABLE 1SRCH_WIN_ASRCH_WIN_ASRCH_WIN_NSRCH_WIN_NWindowSRCH_WIN_NGHBWindowSRCH_WIN_NGHBSizeR SRCH_WIN_RSizeR SRCH_WIN_R(PNCF_SRCH_WIN_N(PN Chips)CF_SRCH_WIN_NChips)048601698028101003101113041412160520132266281432074015452                Neighbor Set: If a flag for a different neighbor search window is set, the search window size for each pilot in the neighbor set is the number of PN chips specified in Table 1, corresponding to search window size parameter associated with the pilot being searched. If the flag is not set, the search window size for each pilot in the neighbor set is the same and is equal to the number of PN chips specified in Table 1 corresponding to SRCH_WIN_NS. The mobile centers the search window for each pilot in the neighbor set around the pilot's PN sequence offset, using timing defined by the mobile's time reference.        Remaining Set: The search window size for each pilot in the remaining set is the number of PN chips specified in Table 1 corresponding to SRCH_WIN_RS. The mobile centers the search window for each pilot in the remaining set around the pilot's PN sequence offset, using timing defined by the mobile's time reference. The mobile searches for remaining set pilots whose pilot PN sequence offset indices are equal to integer multiples of the pilot increment.        Candidate Frequency Search Set: If the flag for candidate frequency is set, the search window size for each pilot in the candidate frequency search shall be the number of PN chips specified in Table 1, corresponding to SRCH_WIN_NGHBR associated with the pilot being searched. If the flag is not set, the search window size for each pilot in the candidate frequency search set shall be the number of PN chips specified in Table 1 corresponding to CF_SRCH_WIN_N. The mobile centers the search window for each pilot in the Candidate Frequency Search Set around the pilot's PN sequence offset using timing defined by the mobile's time reference.        
C. Time to Search
Each phone manufacturer has its own way of implementing a search strategy. In all strategies, the time to search a specific pilot depends on the window size and the hardware of the searcher. Given certain hardware, the time to search a pilot is linearly proportional to the search window size. Reducing the search window size will result in a substantial reduction in searching time. Using current searching procedures, the window sizes are mostly determined by the size of the coverage area of a given cell. A cell is the geographical area covered by a base station for communication with a mobile. Four such cells are shown in FIG. 1. Regardless of the location of the mobile in the serving cell, current search windows are sized to correspond to the worst case scenarios. That is, they are sized to correspond to a mobile located at the greatest distance from the base station but within the cell.
On the paging or traffic channels, the mobile centers its search window for each pilot in the neighbor set around the pilot's PN sequence offset, using timing established by the mobile's time reference. The mobile's time reference is defined as the earliest arrived and usable path. The worst case scenario determines the search window size. For example, FIG. 1 shows four adjacent cells 102, 104, 106, and 108 in a wireless system 100, each having a pilot designated PN1, PN2, PN3, and PN4, respectively. The search window size for pilot PN1 is determined based on a mobile located at point A. However, the same search window is used even if the mobile is at point B. This results in a waste of valuable searcher resources since it does not consider the location of the mobile within cell 104. If the mobile is at point B, the search window should be reduced in size relative to the search window required for a mobile located at point A.
D. Location Methods
Many techniques are being considered to provide for automatic location capability for mobiles. One technique involves measuring the time difference of arrival of signals from a number of cell sites. These signals are “triangulated” to extract location information. This technique requires a high concentration of cell sites and/or an increase in the transmission power of the sites to be effective because typical CDMA systems require each mobile to transmit with only enough signal power to reach the closest cell site. This triangulation requires communication with at least three sites, requiring an increase in the concentration of cell sites or the signal power of each mobile station would have to be increased. Another approach involves the addition of GPS (Global Positioning System) functionality to a mobile. This approach requires a line-of-sight to four satellites, is somewhat slow, but is the most accurate approach for locating a mobile.
A third approach sends aiding information to the mobile indicating in which frequency range the mobile should look for a GPS carrier. Most GPS receivers use what is known as a GPS satellite almanac to minimize a search performed by the receiver in the frequency domain for a signal from a visible satellite. The almanac is a 15,000 bit block of coarse ephemeris and time model data for the entire constellation. The information in the almanac regarding the position of the satellite and the current time of day is approximate only. Without an almanac, the GPS receiver must conduct the widest possible frequency search to acquire a satellite signal. Additional processing is required to attain additional information that will aid in acquiring other satellites. The signal acquisition process can take several minutes due to the large number of frequency bins that need to be searched. Each frequency bin has a center frequency and predefined width. The availability of the almanac reduces the uncertainty in satellite Doppler and therefore the number of bins that must be searched. The satellite almanac can be extracted from the GPS navigation message or sent on the down forward link from the satellite to the mobile as a data or signaling message. On receipt of this information, the mobile performs GPS signal processing to determine its location.
What is needed is a method and apparatus can use the location information for a mobile in conjunction with pilot search techniques to improve the speed by which a mobile can search all pilots on an assigned frequency while the mobile is in control of the traffic channel. The invention should be able to utilize information about the physical location of the mobile to determine the search window size for each pilot in the neighbor and candidate sets.